Few strategies to prevent disease progression and disability in the setting of knee osteoarthritis (OA) exist, large part due to limited knowledge of the factors responsible for these outcomes. This deficiency of knowledge not only hinders development of novel strategies that target these factors, it also clouds our ability to identify individuals who are unlikely to benefit from a given prevention approach. The broad goal of our work is to identify the factors that contribute to OA disease progression and functional status decline the goal of the proposed study is to examine the relationship of 3 local joint-organ factors with 2-year disease progression and functional decline in a cohort of 300 subjects with knee OA: strength, within subsets of OA knees; medial and lateral laxity; and limb position sense during a locomotor task, ergometer pedaling. Progression will be assessed using both radiographic and MRI-based measures, and functional decline using observed and self-report measures The proposed infrastructure. Since 1997, we have been studying a cohort of subjects with knee OA. In this natural history study (MAK, the study of Mechanical Factors in Arthritis of the Knee), time 0 and 1.5 year visits are completed and 3 year visits are ongoing. The ongoing MAK study includes radiographic and functional assessments, but no MRI. Support for components of the next phase of this study to occur 2002-2007, including MRI evaluations, has been procured, substantially reducing the cost required to examine the questions posed in the current proposal. The 2 visits of the proposed study will occur at 5 (the first of the proposed study and 7 years on the MAK timeline. Strength, laxity, and joint position sense may each influence both structural and functional outcome of as vv t knee 0A. I He proposed study will provide important information regarding these factors that will be directly applicable to the development of strategies to prevent disease progression and disability.